This invention relates generally to cathodic protection systems and more particularly to cathodic protection systems for steel-in-concrete structures.
In order to protect steel-in-concrete structures, such as roadways or bridges having a concrete base layer in which steel reinforcing bars are located, various cathodic protection systems have been utilized. In all such systems, an anode or a string of anodes is either laid on the concrete or embedded in it. The anode(s) is(are) connected in a circuit containing a rectifier and the steel reinforcing bars. The current from the rectifier is sent through the circuit wiring to the anode at which point is passes through the concrete itself to the reinforcing bars and from there through a negative return cable to the rectifier.
One type of cathodic protection system is sometimes referred to as an asphalt overlay system. That system basically comprises disposing a plurality of flexibly interconnected, cast iron anodes on the concrete deck or base to be protected and cementing them in place thereon. The anodes are connected (wired) to a rectifier controller. Over the top of the anodes an asphalt layer is laid. That layer usually includes a conductive carbon particulate material (commonly referred to as "coke breeze") used as an aggregate. Disposed over the coke breeze asphalt layer is a conventional asphalt layer. While that cathodic protection system is generally suitable for its purposes, it nevertheless exhibits certain disadvantages, such as weight, replaceability, limited fields of use (e.g., horizontal surfaces) and relatively high cost.
An alternative to the asphalt overlay system are so-called "non-overlay" systems. These systems were developed to overcome the problems of the overlay system and basically consists of two types, namely, the so-called "non-overlay with conductive concrete" system, and the so-called "non-overlay with conductive paint" system. In the non-overlay systems, the anode utilized is frequently a small diameter, e.g., one-eighth inch or less, platinum coated wire.
In one non-overlay system, a series of saw slots are cut in the concrete base layer at spaced locations and a respective platinum wire anode is disposed in each of the saw slots. Those slots are thereafter filled with a conductive concrete grout. The anodes are connected to the rectifier so that D-C. current is discharged from the conductive slots, which thus act as anodes. The current then passes through the concrete to the reinforcing bars and back to the rectifier. While the saw-slot mounted anode, non-overlay system has advantages over the overlay system, it nevertheless suffers from various disadvantages, such as a relatively high cost of installation (the saw slots must be cut in the concrete, cleaned of all debris and then platinum anodes placed in the slots and the conductive ground placed in the slots containing the anodes).
An alternative to the saw-slot non-overlay system has been developed and basically consists of pre-casting the platinum wire anode in a conductive polymer strip approximately one and one-half inches width and three-quarter inch height, then placing the pre-cast polymer anode in place on the surface of the concrete base. While this technique has obviated the necessity of cutting the saw slots and filling them, the formation of such pre-cast anodes is somewhat difficult and they are susceptible to breakage in handling.
Another alternative to a non-overlay system comprises placing the wire anode directly on the surface of the concrete base layer, providing forms (molds) on either side thereof and pouring a polymer concrete into the space between the forms to encapsulate the anode therein. Such a technique (called a "poured-in-place" technique), while offering certain advantages over the use of pre-cast anodes or saw slot disposed anodes, nevertheless is difficult and expensive to accomplish since it requires the use of forms of some sort which have to be placed adjacent the anode to form the polymer concrete strip and then removed after the formation of the strip. Thus, for long structures to be protected, thousands of feet of forms could be required. Moreover, the prior art poured-in-place methods of forming a non-overlay system exhibits the disadvantage that the poured-in-place polymer concrete strip formed thereby is relatively thick, e.g., one-half inch. Hence, if an overlay layer were needed to be placed over the concrete base layer the overlay would have to be thick to cover the poured-in-place concrete strip.
For cathodically protecting some installations, but not roadways, a non-overlay, conductive paint cathodic protection system was developed. That system basically consists of applying a conductive paint (a mastic) completely over the surface of the concrete layer having the reinforcing bars therein. Thereafter, a series of the small diameter platinum wires are attached to the concrete paint layer utilizing strips of self-adhesive fiberglas mesh. The mesh is then covered with another layer of conductive mastic. The anode system is completed and covers the entire concrete surface. A cosmetic acrylic paint can then be put over the mastic, if desired.
While this system is generally suitable for its intended purposes, and offers advantages over various other cathodic protection systems, it still leaves something to be desired. In this connection such a system cannot be used if a concrete overlay layer is to be placed over the base layer, nor may the mastic material provide sufficient mass to ensure long system life.